Keflex Mechanism of Action

A semisynthetic cephalosporin antibiotic.
Microbiology: In vitro tests demonstrate that the cephalosporins are bactericidal because of their inhibition of cell wall synthesis. Cephalexin is active against the following organisms in vitro: β-Haemolytic streptococci; Staphylococci, including coagulase-positive, coagulase-negative and penicillinase-producing strains; Streptococcus (Diplococcus) pneumoniae; Escherichia coli; Proteus mirabilis; Klebsiella sp, Haemophilus influenzae; Moraxella (Branhamella) catarrhalis.
Note: Most strains of enterococci (Enterococcus faecalis [formerly Streptococcus faecalis]) and a few strains of staphylococci are resistant to cephalexin. It is not active against most strains of Enterobacter sp, Morganella morganii (formerly Proteus morganii) and Proteus vulgaris. It has no activity against Pseudomonas or Acinetobacter calcoaceticus (formerly Mima and Herellea sp). When tested by in vitro methods, staphylococci exhibit cross-resistance between cephalexin and methicillin-type antibiotics.
Susceptibility Testing: Quantitative methods that require measurement of zone diameters give the most precise estimates of antibiotics susceptibility. One such procedure has been recommended for use with disks for testing susceptibility to cephalothin. Interpretations correlate zone diameters of the disk test with MIC values of cephalexin. With this procedure, a report from the laboratory of "resistant" indicates that the infecting organism is not likely to respond to therapy. A report of "intermediate susceptibility" suggests that the organism would be susceptible if the infection is confined to the urine, in which high antibiotic levels can be obtained, or if high dosage is used in other types of infection.
Pharmacokinetics: Cephalexin is acid-stable and may be given without regard to meals. It is rapidly absorbed after oral administration. Following doses of 250-mg, 500-mg and 1-g, average peak serum levels of approximately 9, 18 and 32 mg/L, respectively, were obtained at 1 hr. Measurable levels were present 6 hrs after administration. Cephalexin is excreted in the urine by glomerular filtration and tubular secretion. Studies showed that >90% of the drug was excreted unchanged in the urine within 8 hrs. During this period, peak urine concentrations following the 250-mg, 500-mg and 1-g doses were approximately 1000, 2200 and 5000 mg/L, respectively.